Hitherto, the flux method has been employed for producing a Group III nitride-based compound semiconductor crystal, and conventional production methods based on the flux method are disclosed in, for example, Patent Documents 1 to 5 described below.
A sapphire substrate, which is much less expensive than a GaN self-standing substrate, has significantly high industrial value. In conventional semiconductor production methods based on the flux method, particularly when a sapphire substrate is employed as a crystal growth substrate, the employed sapphire substrate has a C-plane main surface (hereinafter may be referred to as a C-plane substrate), as disclosed in, for example, Patent Document 1. Generally, as disclosed in, for example, Patent Document 2, a GaN layer serving as a seed crystal is formed on the main plane of the substrate through MOVPE or a similar technique, and the thus-coated sapphire C-plane substrate is employed as a crystal growth substrate.
Meanwhile, when the produced group III nitride-based compound semiconductor crystal is employed as a semiconductor substrate for fabricating optical devices, electronic devices, etc., the substrate preferably has a thickness of 400 μm or more so as to ensure sufficient strength and to allow easy and reliable handling.
Patent Document 1:
    Japanese Patent Application Laid-Open (kokai) No. 2005-194146Patent Document 2:    Japanese Patent Application Laid-Open (kokai) No. 2004-300024Patent Document 3:    Japanese Patent Application Laid-Open (kokai) No. 2004-292286Patent Document 4:    Japanese Patent Application Laid-Open (kokai) No. 2004-168650Patent Document 5:    Japanese Patent Application Laid-Open (kokai) No. 2003-292400